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Known and Probable Human Carcinogens

Introduction

Many people worry that substances or exposures in their environment may cause cancer. As part of the American Cancer Society's role in informing and educating people about cancer and its possible causes, this document provides lists of substances and exposures that are known or suspected to cause cancer. The lists below have been developed by two highly respected agencies – the International Agency for Research on Cancer (IARC) and the US National Toxicology Program (NTP). Some related information is included on how these and other agencies and groups test and classify possible carcinogens.

The American Cancer Society does not keep detailed information on each of the exposures on these lists. If you are looking for more in-depth information on a particular item on these lists, please refer to the agencies in the "Additional resources" section of this document.

What is a carcinogen?

Cancer is caused by changes in a cell's DNA – its genetic "blueprint." Some of these changes may be inherited from our parents, while others may be caused by outside exposures, which are often referred to as environmental factors. Environmental factors can include a wide range of exposures, such as:

Substances and exposures that can lead to cancer are called carcinogens. Some carcinogens do not affect DNA directly, but lead to cancer in other ways. For example, they may cause cells to divide at a faster than normal rate, which could increase the chances that DNA changes will occur.

Carcinogens do not cause cancer in every case, all the time. Substances labeled as carcinogens may have different levels of cancer-causing potential. Some may cause cancer only after prolonged, high levels of exposure. And for any particular person, the risk of developing cancer depends on many factors, including how they are exposed to a carcinogen, the length and intensity of the exposure, and the person's genetic makeup.

How do researchers determine if something is a carcinogen?

Testing to see if something can cause cancer is often difficult. It is not ethical to test a substance by exposing people to it and seeing if they get cancer from it. That’s why scientists must use other types of tests, which may not always give clear answers.

Lab studies

Scientists get much of their data about whether something might cause cancer from lab studies in cell cultures and animals. There are far too many substances (both natural and man-made) to test each one in lab animals, so scientists use what is already known about chemical structures, results from other types of lab tests, the extent of human exposure, and other factors to select chemicals for testing. For example, they can often get an idea about whether a substance might cause a problem by comparing it to similar chemicals that have already been studied.

Although lab studies alone can't always predict if a substance will cause cancer in people, virtually all known human carcinogens that have been adequately tested also cause cancer in lab animals. In many cases, carcinogens are first found to cause cancer in lab animals and are later found to cause cancer in people.

Most studies of potential carcinogens expose the lab animals to doses that are much higher than common human exposures. This is so that cancer risk can be detected in relatively small groups of animals. It is not always clear if the results from animal studies will be the same for people as they are normally exposed to a substance. For example, the effects seen in lab studies with very high doses of a substance may not be the same at much lower doses, or the effects of a substance when it is inhaled may not be the same as if it is applied to the skin. Also, the bodies of lab animals and humans don't always process substances in the same way.

But for safety reasons, it is usually assumed that exposures that cause cancer at larger doses in animals may also cause cancer in people. It isn't always possible to know how the exposure dose might affect risk, but it is reasonable for public health purposes to assume that lowering human exposure will reduce risk.

Studies in people

Another important way to identify carcinogens is through epidemiologic studies, which look at human populations to determine which factors might be linked to cancer. These studies also provide useful information, but they have their limits. Humans do not live in a controlled environment. People are exposed to many substances at any given time, including those they encounter at work, school, or home; in the food they eat; and in the air they breathe. It's very unlikely they know exactly what they've been exposed to or that they would be able to remember all of their exposures if asked by a researcher. And there are usually many years (often decades) between exposure to a carcinogen and the development of cancer. Therefore, it can be very hard to definitely link any particular exposure to cancer.

By combining data from both types of studies, scientists do their best to make an educated assessment of a substance's cancer-causing ability. When the evidence is conclusive, the substance is labeled as a carcinogen. When the available evidence is compelling but not felt to be conclusive, the substance may be considered to be a probable carcinogen. But in some cases there simply isn't enough information to be certain one way or the other.

Who determines how carcinogens are classified?

Several national and international agencies are responsible for determining the cancer-causing potential of different substances.

International Agency for Research on Cancer

The International Agency for Research on Cancer (IARC) is part of the World Health Organization (WHO). Its major goal is to identify causes of cancer. The most widely used system for classifying carcinogens comes from the IARC. In the past 30 years, the IARC has evaluated the cancer-causing potential of more than 900 likely candidates, placing them into one of the following groups:

Group 1: Carcinogenic to humans

Group 2A: Probably carcinogenic to humans

Group 2B: Possibly carcinogenic to humans

Group 3: Unclassifiable as to carcinogenicity in humans

Group 4: Probably not carcinogenic to humans

Perhaps not surprisingly, based on how hard it can be to test these candidate carcinogens, most are listed as being of probable, possible, or unknown risk. Only a little over 100 are classified as “carcinogenic to humans.”

National Toxicology Program

The National Toxicology Program (NTP) is formed from parts of several different US government agencies, including the National Institutes of Health (NIH), the Centers for Disease Control and Prevention (CDC), and the Food and Drug Administration (FDA). The NTP updates its Report on Carcinogens (RoC) every few years.

The Report on Carcinogens identifies 2 groups of agents:

“Known to be human carcinogens”

“Reasonably anticipated to be human carcinogens”

The current version of the RoC lists about 240 substances and exposures. Unlike the IARC’s list, the RoC does not list substances that have been studied and found not to be carcinogens.

Environmental Protection Agency

The US Environmental Protection Agency (EPA) maintains the Integrated Risk Information System (IRIS), an electronic database that contains information on human health effects from exposure to certain substances in the environment. The EPA uses a rating system similar to that of IARC when describing the cancer-causing potential of a substance:

Group A: Carcinogenic to humans

Group B: Likely to be carcinogenic to humans

Group C: Suggestive evidence of carcinogenic potential

Group D: Inadequate information to assess carcinogenic potential

Group E: Not likely to be carcinogenic to humans

Other agencies and groups

Other federal agencies, such as the CDC’s National Institute for Occupational Safety and Health (NIOSH), the Food and Drug Administration (FDA), and the National Cancer Institute may comment on whether a substance or exposure may cause cancer and/or what levels of exposure to a particular substance might be considered acceptable.

Some state agencies also keep lists of known or probable carcinogens. For example, the California Environmental Protection Agency (CalEPA) maintains a list of “chemicals known to the state to cause cancer or reproductive toxicity.” (Much of this list is based on the IARC and NTP lists below.)

The American Cancer Society’s role

The American Cancer Society (ACS) contributes in many ways to evaluating how environmental factors affect a person's likelihood of developing cancer, including:

Conducting epidemiologic research on the causes of cancer

Funding laboratory and epidemiologic research at universities and other institutions that study environmental causes of cancer

Advocating for environmental health on local, state, and federal levels

Informing the public about environmental factors that affect cancer risk and how to decrease their risk of developing cancer

In most cases, the ACS does not directly evaluate whether a certain substance or exposure causes cancer. Instead, the ACS looks to national and international organizations such as the NTP and IARC, whose mission is to evaluate environmental cancer risks based on evidence from laboratory and human research studies.

Some important points about the IARC and NTP lists here

The IARC and NTP act independently but have studied many of the same agents, so many known or suspected carcinogens appear on both lists. But because an agent appears on one and not on the other does not necessarily mean there is a controversy, as one agency may not have evaluated it.

Unfortunately, many of the substances and exposures on the lists below can often go by different names. This can make it hard to find a particular substance on one or both of these lists, which are in alphabetical order and may not always use the most common term.

These lists include only those agents that have been evaluated by the agencies. These agencies tend to focus on substances and exposures most likely to cause cancer, but there are many others that have not been studied fully yet.

Most of the agents on the lists have been linked only with certain kinds of cancer, not all types. For more detailed information, refer to the specific monographs or reports published by the agencies (available on their websites).

The lists themselves say nothing about how likely it is that an agent will cause cancer. Carcinogens do not cause cancer at all times, under all circumstances. Some may only be carcinogenic if a person is exposed in a certain way (for example, swallowing it as opposed to touching it). Some may only cause cancer in people who have a certain genetic makeup. Some of these agents may lead to cancer after only a very small exposure, while others might require intense exposure over many years. Again, you should refer to the agencies’ reports for specifics.

Even if a substance or exposure is known or suspected to cause cancer, this does not necessarily mean that it can or should be avoided at all costs. For example, estrogen is a known carcinogen that occurs naturally in the body. Also, exposure to ionizing radiation is known to cause cancer, with increased risks even at low levels of exposure. Yet there is no way to completely prevent exposure to natural sources of radiation such as cosmic radiation from the sun or radon in soil. These lists also include many commonly used medicines, particularly some hormones and drugs used to treat cancer. For example, tamoxifen increases the risk of certain kinds of uterine cancer but can be very useful in treating some breast cancers, which may be more important for some women. If you have questions about a medicine that appears on one of these lists, be sure to ask your doctor.

Looking at the list below can tell you whether or not something may increase your risk of cancer, but it is important to try to get an idea of how much it might increase your risk. It is also important to know what your risk is to begin with. Many factors can enter into this, including your age, gender, family history, and lifestyle factors (tobacco and alcohol use, weight, diet, physical activity level, etc.). As noted above, the type and extent of exposure to a substance may also play a role. You should consider the actual amount of increased risk when deciding if you should limit or avoid an exposure.

Known human carcinogens

International Agency for Research on Cancer
Group 1: Carcinogenic to humans

Radionuclides, alpha-particle-emitting, internally deposited (Note: Specific radionuclides for which there is sufficient evidence for carcinogenicity to humans are also listed individually as Group 1 agents)

Radionuclides, beta-particle-emitting, internally deposited (Note: Specific radionuclides for which there is sufficient evidence for carcinogenicity to humans are also listed individually as Group 1 agents)

Radium-224 and its decay products

Radium-226 and its decay products

Radium-228 and its decay products

Radon-222 and its decay products

Rubber manufacturing industry

Salted fish (Chinese-style)

Schistosoma haematobium (infection with)

Semustine (methyl-CCNU)

Shale oils

Silica dust, crystalline, in the form of quartz or cristobalite

Solar radiation

Soot (as found in workplace exposure of chimney sweeps)

Sulfur mustard

Tamoxifen (Note: There is also conclusive evidence that tamoxifen reduces the risk of contralateral breast cancer in breast cancer patients)

2,3,7,8-Tetrachlorodibenzo-para-dioxin

Thiotepa

Thorium-232 and its decay products

Tobacco, smokeless

Tobacco smoke, secondhand

Tobacco smoking

ortho-Toluidine

Treosulfan

Trichloroethylene

Ultraviolet (UV) radiation, including UVA, UVB, and UVC rays

Ultraviolet-emitting tanning devices

Vinyl chloride

Wood dust

X- and Gamma-radiation

National Toxicology Program 13th Report on Carcinogens
“Known to be human carcinogens”

Aflatoxins

Alcoholic beverage consumption

4-Aminobiphenyl

Analgesic mixtures containing phenacetin

Aristolochic acids

Arsenic and inorganic arsenic compounds

Asbestos

Azathioprine

Benzene

Benzidine

Beryllium and beryllium compounds

Bis(chloromethyl) ether and technical-grade chloromethyl methyl ether

1,3-Butadiene

1,4-Butanediol dimethylsulfonate (also known as busulfan)

Cadmium and cadmium compounds

Chlorambucil

1-(2-Chloroethyl)-3-(4-methylcyclohexyl)-1-nitrosourea (MeCCNU)

Chromium hexavalent compounds

Coal tar pitches

Coal tars

Coke oven emissions

Cyclophosphamide

Cyclosporin A

Diethylstilbestrol (DES)

Dyes metabolized to benzidine

Erionite

Estrogens, steroidal

Ethylene oxide

Formaldehyde

Hepatitis B virus

Hepatitis C virus

Human papilloma viruses: some genital-mucosal types

Melphalan

Methoxsalen with ultraviolet A therapy (PUVA)

Mineral oils (untreated and mildly treated)

Mustard gas

2-Naphthylamine

Neutrons

Nickel compounds

Oral tobacco products

Radon

Silica, crystalline (respirable size)

Solar radiation

Soots

Strong inorganic acid mists containing sulfuric acid

Sunlamps or sunbeds, exposure to

Tamoxifen

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD); "dioxin"

Thiotepa

Thorium dioxide

Tobacco smoke, environmental

Tobacco, smokeless

Tobacco smoking

o‑Toluidine

Vinyl chloride

Ultraviolet (UV) radiation, broad spectrum

Wood dust

X-radiation and gamma radiation

Probable carcinogens

International Agency for Research on Cancer
Group 2A: Probably carcinogenic to humans

*Inclusion on this list does not imply endorsement by the American Cancer Society.

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References

International Agency for Research on Cancer (IARC). Agents Classified by the IARC Monographs, Volumes 1 – 112. 2015. Accessed at http://monographs.iarc.fr/ENG/Classification/ClassificationsGroupOrder.pdf on March 26, 2015.

US Department of Health and Human Services. Public Health Service, National Toxicology Program. Report on Carcinogens, Thirteenth Edition. 2014. Accessed at http://ntp.niehs.nih.gov/pubhealth/roc/roc13/index.html on October 2, 2014.